Around miles 8 to 10, the Illinois Marathon course runs through Meadowbrook Park in Urbana. The park is beautiful: a tallgrass prairie restoration with sculpture gardens, winding paths, and scenic bridges. It's one of the course highlights visually, and it's also the most congested section of the race.
The paved paths through Meadowbrook Park are roughly six people wide or less, particularly over the bridges. At this point in the race, the full marathon and half marathon fields are still running together, which means several thousand runners are funneling onto a narrow path system. If you're running slower than about a 9:00 to 10:00 per mile pace, the congestion can be significant. Runners have reported having to slow down, weave, or wait briefly at the narrowest points.
For faster runners (sub-4:00 marathon pace), the congestion is minimal because the field has already spread out ahead of you. For mid-pack and back-of-pack runners, the bottleneck is real and can cost time. The race organizers have adjusted the course routing over the years to address this (moving the park section from mile 4 to mile 8 after the inaugural year), but the fundamental constraint is the path width.
The practical advice: if you're running near the BQ cutoff and every minute counts, be aware of the Meadowbrook Park section and stay near the front of the pack through the first 7 miles to enter the park with fewer runners around you. If you're running for the experience rather than a specific time, relax and enjoy the park. It's genuinely one of the prettier sections of any Midwest marathon course, and the congestion, while annoying, resolves within a mile.